1. Field of the Invention
The present invention relates to a contact type measuring instrument provided with a device for adjusting a probe contact force.
2. Description of the Related Art
To measure the three-dimensional profile of a workpiece on a processing machine is of great importance in performing working operation with high accuracy. As a technique for measuring the profile of a workpiece, there has conventionally been known a probe type profile measuring sensor that measures the three-dimensional profile of a workpiece by bringing the tip end of a profile probe into contact with the surface of the workpiece.
The measuring probe of the probe type profile measuring sensor is configured so that the measuring probe is supported by a bearing or an air slide so as to be movable in the axial direction of a probe shaft, and is urged toward an object to be measured by a spring or a pneumatic pressure to apply a preload. Therefore, the contact pressure varies according to a change in probe position, the contact pressure is too high, and it is difficult to freely adjust the contact pressure.
Japanese Patent Application Laid-Open No. 2007-155440 describes a contact type measuring instrument in which an urging space is provided in the bearing part of a probe, and compressed air is supplied to this space to adjust the contact force of the probe that is brought into contact with an object to be measured. This contact type measuring instrument includes an urging device for moving a probe shaft toward the object to be measured by supplying compressed air to the urging space and by giving an urging force to the probe shaft, a minute force measuring device for detecting a minute contact pressure between a contact and the object to be measured, a control section for controlling the urging force of the urging device based on the contact pressure detected by the minute force measuring device, and a displacement measuring device for measuring the position of the contact that is in contact with the object to be measured. This contact type measuring instrument requires control of minute pneumatic pressure to adjust the minute contact force, and is easily affected by very small pulsation of pneumatic pressure.
Besides the above-described contact type measuring instrument in which compressed air is supplied to the urging space provided in the bearing part of the probe, there has been known a contact type measuring instrument in which the contact force of probe is adjusted by using an electromagnet and a permanent magnet. For the contact type measuring instrument of this type, to adjust the contact force, electric currents supplied to respective electromagnets at the front and rear of the probe must be controlled separately, so that a supplementary device for current control is needed. In addition, the electromagnet easily generates heat, and the probe is expanded by this heat generation, so that it is difficult to use this measuring instrument as a measuring instrument that requires submicron or less minute measurement.
In the contact type measuring instrument using a probe, the contact force of the probe with respect to the object to be measured must be adjusted according to the travel speed of the probe and the shape and material of the object to be measured. Also, in the case where the probe is installed in the vertical direction, an influence of the weight of the probe itself must be excluded to obtain a minute contact force.